Information recording medium and method of manufacturing resinous substrate for use in the recording medium

ABSTRACT

An information recording medium comprising a substrate having a recording surface provided with emboss pits or guiding grooves, a reflective film formed on the recording surface of the substrate, and a first protective film formed on the reflective film. This information recording medium is featured in that both sides of the information recording medium are constituted by a first surface provided with the protective film and by a second surface formed opposite to the first surface, and that an irradiated light beam is irradiated through the first surface, a recorded information being reproduced based on changes in light intensity of the reflected light beam. The distance between the recording surface of the substrate and the light incident surface is smaller than a thickness of the substrate, and a surface roughness “R” of the light incident surface meets a relationship represented by the following formula (1):
 
R≦λ/(8n)   (1)
         wherein λ is a wavelength of the light beam, and n is a refractive index of the first protective film to a light having the wavelength λ.

This application is a division divisional reissue application of U.S.application Ser. No. 12/028,491, filed Feb. 8, 2008. U.S. applicationSer. No. 12/028,491 is a reissue application of U.S. application Ser.No. 09/657,566, filed Sep. 8, 2000, now U.S. Pat. No. 6,465,069. U.S.Pat. No. 6,465,069 is a divisional of Application U.S. application Ser.No. 09/283,161, filed Apr. 1, 1999. Allowed as, now U.S. Pat. No.6,159,572 and for which priority is claimed under 35 U.S.C. §121 and 35U.S.C. §251. This application is based upon and claims the benefit ofpriority under 35 U.S.C. §119 from the prior Japanese Patent ApplicationNo. 10-091422, filed Apr. 3, 1998. Notice: More than one reissueapplication has been filed for the reissue of U.S. Pat. No. 6,465,069.The reissue applications are application Ser. Nos. 12/476,449,12/476,441, 12/476,434, 12/476,428, 12/476,375, 12/476,456, 12/476,607,12/476,627, 12/476,685, 12/476,703, and 12/476,745, all of which aredivisional reissues of reissue application Ser. No. 12/028,491.

BACKGROUND OF THE INVENTION

This invention relates to an information recording medium and to amethod of manufacturing a resinous substrate to be employed for theinformation recording medium. In particular, this invention relates to asurface recording/reproducing type information recording medium and to amethod of manufacturing a resinous substrate to be employed for such aninformation recording medium.

An ordinary optical disk such as CD, CD-ROM, etc. is constructed suchthat emboss pits are formed in conformity with the recorded data on oneof the surfaces of a transparent substrate having a thickness of 1.2 mm,the emboss pits being covered thereon by a reflective film made of Alfor example. The information recorded in the CD constructed in thismanner can be reproduced by irradiating a converging beam onto theemboss pits from a surface of the transparent substrate which isopposite to the other surface where the reflective film is formed.

On the other hand, an optical disk such as DVD, DVD-ROM where therecording density is highly enhanced is constructed such that fineremboss pits than those of the CD are formed on one of the surfaces of atransparent substrate having a thickness of 0.6 mm, the emboss pitsbeing also covered thereon by a reflective film made of Al for example.The information recorded on the recording surface of the diskconstructed in this manner can be reproduced in the same manner as thatof the CD, i.e. by irradiating a converging beam onto the emboss pitsfrom a surface of the transparent substrate which is opposite to theother surface where the reflective film is formed.

As for the material for the substrate having a thickness of 0.6 mm, PC(polycarbonate) which is a transparent resin is generally employed. ThisPC substrate having a thickness of 0.6 mm however is not sufficient inmechanical property, resulting in the warping of the substrate as it isemployed singly. Therefore, in order to prevent the substrate from beingwarped, a couple of PC substrates each having a thickness of 0.6 mm aresuperimposed each other with the recording surface being directedinside, thus forming a disk having a total thickness of 1.2 mm, therebyensuring the mechanical property thereof.

The reason for setting the substrate of DVD to 0.6 mm is to secure thetilt margin of the disk. When the density of track pitch or pit isincreased, the margin of the inclination or so-called tilt of the diskis caused to decrease. Although it may be possible to secure the tiltmargin by decreasing the thickness of the substrate from 1.2 mm to 0.6mm, it will inevitably result in a deterioration of the mechanicalproperty thereof.

Under the circumstances, there has been proposed, with a view to securethe mechanical strength of the disk while decreasing the thickness ofthe substrate, an idea of thickening the central portion of the diskthereby to ensure the mechanical strength thereof (Japanese PatentUnexamined Publication H9-204686). However, it is required, for ensuringa sufficient mechanical strength, to make the thickness of the signalrecording region of the substrate at least 0.6 mm. Further, there isalso reported an idea of making the thickness of the substrate to rangefrom 0.1 mm to 0.6 mm (Japanese Patent Unexamined PublicationH9-204688). However, the thickness of a protective substrate forsustaining the recording film as well as the film thickness of thereflective film are not referred to in the idea, thus making itdifficult to practice in the actual application thereof.

U.S. Pat. No. 5,757,733 teaches an information recording mediumcomprising a covering layer formed on the light beam incident side, anda flat substrate sustaining a recording film. However, this coveringlayer is simply referred to as having a thickness of 0.6 to 1.0 mm.

For the purpose of further increasing the capacity of the disk bycompacting the recording density, it is more effective, in view ofassuring the tilt margin, to make the thickness of the substrate as thinas possible. However, when the thickness of the substrate becomes lessthan 0.6 mm, it becomes difficult to secure the mechanical strengththereof even if a couple of substrates are superimposed each other.

Moreover, this superimposition of a couple of substrates is accompaniedwith the problems that it not only requires the employment of anadhesive but also makes the manufacturing process thereof morecomplicated.

BRIEF SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide aninformation recording medium which is capable of securing a sufficienttilt margin and a sufficient mechanical strength even if the recordingdensity is further increased.

Another object of the present invention is to provide a method ofmanufacturing a resinous substrate which is adapted to be employed forsuch an information recording medium.

Namely, according to this invention, there is provided an informationrecording medium, which comprises a substrate having a recording surfaceprovided with emboss pits or guiding grooves; a reflective film formedon the recording surface of the substrate; and a first protective filmformed on the reflective film;

-   -   wherein both sides of the information recording medium are        constituted by a first surface constituting an uppermost surface        on the first protective film and by a second surface formed        opposite to the first surface;    -   the first surface is constituted as a light incident surface,        thereby allowing an irradiated light beam to enter and reflect        through the first surface, a recorded information being        reproduced based on changes in light intensity of the reflected        light beam; and wherein        -   a distance between the recording surface of the substrate            and the light incident surface is smaller than a thickness            of the substrate, and a surface roughness “R” of the light            incident surface meets a relationship represented by the            following formula (1):            R≦λ/(8n)   (1)            -   where λ is a wavelength of the light beam; and n is a                refractive index of the first protective film to a light                having the wavelength λ.

This invention further provides an information recording medium, whichcomprises;

-   -   a substrate having a couple of recording surfaces facing to each        other and each provided with emboss pits or guiding grooves;    -   a couple of reflective films each formed on each of the        recording surfaces of the substrate;    -   a couple of first protective films each formed on each of the        reflective film; and    -   a couple of light incident surfaces each constituting an        outermost surface;    -   wherein a light beam to be irradiated is designed to be entered        and reflected through the couple of light incident surfaces, a        recorded information being reproduced based on changes in light        intensity of the reflected light beam;    -   a distance between one of the light incident surfaces of the        information recording medium to the other is not more than 1.2        mm;    -   a distance between the recording surface of the substrate and        the light incident surface formed over the recording surface is        smaller than a thickness of the substrate; and wherein    -   a surface roughness “R” of the couple of light incident surfaces        meets a relationship represented by the following formula (1):        R≦λ/(8n)   (1)        -   where λ is a wavelength of the light beam; and n is a            refractive index of the first protective film to a light            having the wavelength λ.

This invention further provides an information recording medium, whichcomprises;

-   -   a surface substrate having a light incident surface and an        emboss pit surface provided with emboss pits and facing the        light incident surface;    -   a first reflective film formed on the emboss pit surface of the        surface substrate;    -   a supporting substrate having an emboss pit surface provided        with emboss pits and a substrate surface facing the emboss pi t        surface;    -   a second reflective film formed on the emboss pit surface of the        supporting substrate; and    -   a transparent adhesive layer bonding the first reflective film        and the second reflective film;    -   wherein a first record surface is constituted by the emboss spit        surface of the surface substrate and the first reflective film;    -   a second record surface is constituted by the emboss spit        surface of the supporting substrate and the second reflective        film; and wherein    -   a light beam to be irradiated is designed to be entered and        reflected through the light incident surface, a recorded        information being reproduced based on changes in light intensity        of the reflected light beam.

Further, this invention also provides a method of manufacturing aresinous substrate having a first recording surface provided with embosspits or guiding grooves, and a second recording surface facing the firstrecording surface and provided with emboss pits or guiding grooves,which comprises the steps of;

-   -   mounting a first stamper platen for forming a first recording        surface on a first die;    -   mounting a second stamper platen for forming a second recording        surface on a second die;    -   positioning the first die and second die so as to keep a space        therebetween and to arrange the first stamper platen to face the        second stamper platen;    -   filling the space formed between the first die and second die        with a heated and fluidized resinous material;    -   bringing the first die and second die close to each other        thereby adjusting an interval between the first die and second        die to a predetermined distance; and    -   allowing the resinous material filled between the first die and        second die to cool and solidify thereby to obtain a double        surface substrate having a thickness of not more than 1.2 mm.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross-sectional view schematically showing one example of aninformation recording medium according to this invention;

FIG. 2 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIG. 3 is a graph illustrating the dependency of the film thickness of areflective film on the light wavelength;

FIG. 4 is a cross-sectional view schematically showing another exampleof an information recording medium according to this invention;

FIG. 5 is a schematic view of a die for forming a disk substrate to beemployed for the manufacture of the information recording medium shownin FIG. 4;

FIG. 6 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 7 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 8 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 9 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 10 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 11 is a cross-sectional view schematically showing a method ofinjection-molding the information recording medium shown in FIG. 10;

FIGS. 12A to 12D are perspective views illustrating the steps ofmanufacturing the information recording medium shown in FIG. 10;

FIG. 13 is a cross-sectional view schematically showing still anotherexample of an information recording medium according to this invention;

FIG. 14 is a schematic view for illustrating the reflectance and thetransmittance of the recording surface in a 2-ply optical disk;

FIG. 15 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIG. 16 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIG. 17 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIG. 18 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIG. 19 is a graph showing a relationship between the film thickness andreflectance of a reflective film;

FIGS. 20A and 20B are cross-sectional views schematically showing stillanother example of an information recording medium according to thisinvention; and

FIG. 21 is a graph illustrating a relationship between the wavelength oflight beam for recording and reproducing a data and the intensity ofreflected beam in a 2-ply disk.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be further explained in detail with reference to theexamples of this invention.

EXAMPLE 1

FIG. 1 is a cross-sectional view schematically showing one example of anoptical disk according to this example. In the optical disk shown inFIG. 1, one of the surfaces of a disk substrate 1 having a thickness of1.2 mm is formed into a recording surface on which emboss pits 2 areformed in conformity with a recorded data. The thickness of thesubstrate 1 may be suitably selected as long as the thickness in therange of from 0.6 mm to 1.2 mm. The surface of this recording surfaceprovided with the emboss pits 2 is covered with a reflective film 3 andthen with an over-coating consisting of a protective film (firstprotective film) 4. In the case of the optical disk shown in FIG. 1, Thesurface of the protective film 4 is the light incident surface.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 1, a light beam 5 is converged by anobjective lens 6 and then enters through the protective film 4. Thelight beam is then reflected by the reflective film 3, thus producing alight reflection accompanying changes in light intensity, which are thendetected as the recorded data of the emboss pits.

According to the conventional optical disk, light beam enters not fromthe protective film side but from the transparent substrate side, and isreflected by the emboss formed on the surface opposite to the lightincident surface of the substrate, thus enabling the emboss data to beread. By contrast, in the case of the optical disk having a structure asshown in FIG. 1 however, since the data is to be reproduced from theembossed surface formed on the surface of the substrate, the filmthickness of the protective film 4 can be considered as corresponding tothe thickness of the substrate according to the conventional opticaldisk.

According to the optical disk of this invention, the distance from theembossed surface (recording surface) to the light incident surface isdefined as being smaller than the thickness of the substrate 1, thethickness of the protective film 4 is naturally smaller than thethickness of the substrate. Therefore, the optical disk of thisinvention will be hardly influenced by the restriction due to thethickness of the substrate on the tilt margin, thus making it easy toincrease the recording density.

According to the information recording medium of this invention, thesurface roughness “R” of the surface to which a light beam is to beirradiated, i.e. the light incident surface is defined so as to meet arelationship represented by the following formula (1):R≦λ/(8n)   (1)

-   -   wherein λ is a wavelength of the light beam; and n is a        refractive index of the protective film to a light having the        wavelength λ.

It becomes possible, by limiting the surface roughness of the lightincident surface as described above thereby ensuring the flatness of thelight incident surface, to minimize the light-diffracting phenomenon atthe light incident surface. Additionally, if the surface of the opticaldisk is flat as defined above, dust can be hardly attached to thesurface of the optical disk, thus improving a dust adhesion preventiveeffect of the optical disk. Moreover, if the surface of the optical diskis flat as defined above, it is also possible to obtain the effect thatthe optical disk can be prevented from being contacted with theobjective lens. By the way, more preferable surface roughness of thelight incident surface is not higher than a half of λ/(8n), i.e. nothigher than λ/(16n).

It is possible, according to the information recording medium of thisinvention, to further include a second protective film which is to beformed on the first protective film as mentioned hereinafter. In anycase, the outermost surface from which a light beam is to be irradiatedbecomes the light incident surface in the information recording mediumof this invention. For example, when this second protective film isformed on the surface of the first protective film, the surface of thesecond protective film becomes the light incident surface. It is alsorequired in this case that the surface roughness R of the light incidentsurface constituted by the second protective film should meet therelationship represented by the aforementioned formula (1). Further, inthis case also, it is an indispensable requirement for the informationrecording medium of this invention that the distance from the recordingsurface to the light incident surface should be smaller than thethickness of the substrate.

The embossing of the recording surface of the disk substrate is formedwith a size of about λ/(8n) in relative to the wavelength λ of a lightbeam to be employed for the reproduction of a recorded data (wherein nis a refractive index of the surface protective film to a light havingthe aforementioned wavelength λ) The distance from the recording surfaceof the disk substrate to the surface to be irradiated (light incidentsurface) should preferably be at least 10 times higher than themagnitude of the embossing in view of the smoothing of the lightincident surface. Accordingly, the distance from the recording surfaceof the disk substrate to the light incident surface should preferably be5λ/(4n). Further, in view of shortening the wavelength of the light beamand a tilt margin involved in highly enhancing the NA of the objectivelens, the distance from the recording surface of the disk substrate tothe light incident surface should preferably be 0.1 mm or less.

The optical disk shown in FIG. 1 can be manufactured as follows. Firstof all, the reflective film 3 is formed on the recording surface of thedisk substrate 1. In this case, the reflective film 3 can be depositedon the recording surface of the disk substrate 1 by the vapor depositionor sputtering of a material for the reflective film.

The protective film 4 to be placed on the surface of this reflectivefilm 3 can be formed by making use of a conventional ultraviolet-curingresin for instance. Namely, the protective film 4 is at first coated onthe surface of the reflective film 3 by means of a spin coating methodfor instance thereby to form a resin film, and then irradiated withultraviolet rays thereby to cure the resin film, thus forming theprotective film 4. The thickness of this over-coating protective film 4may be in the range of from several microns to several millimeters inpractical view point. More preferably, the thickness of thisover-coating protective film 4 should be not more than 0.6 mm, mostpreferably in the range of 0.0001 to 0.1 mm. Additionally, the thicknessof the protective film 4 should preferably be such that does notoptically interfere with the reflective film 3.

As for the material for the protective film 4, it is not restricted toultraviolet-curing resins, but may be any material as long as it iscapable of allowing a light beam for the reproduction of data to betransmitted therethrough and is stable environmentally and thermally.For example, the protective film 4 may be constituted by a dielectricmaterial. More specifically, the protective film 4 may be formed bymeans of a vacuum deposition method or a sputtering method employingSiO₂, SiO, AlN, Al₂O₃, ZrO₂, TiO₂, Ta₂O₃, ZnS, Si, Ge or a mixturethereof.

This protective film 4 may be omitted provided that the reflective film3 per se is formed of a stable film.

The disk substrate 1 having a thickness of 1.2 mm and being useful forthe manufacture of the optical disk of this example can be manufacturedby means of an injection molding method which is commonly employed forthe manufacture of the conventional CD and DVD. For example, a masterplaten in which information is stored in advance is mounted on one ofthe dies of an injection molding machine, and then an injection moldingis performed after adjusting the space between a couple of dies in sucha way that the thickness of the substrate after the molding thereofbecomes 1.2 mm, thus manufacturing a disk substrate having a thicknessof 1.2 mm.

As mentioned above, according to the conventional optical disk, sincerecorded data is designed to be reproduced by irradiating a light beamfrom the substrate side, the substrate is required to have a capabilityof permitting a light beam for the reproduction of data to transmittherethrough. Whereas, according to the optical disk of this invention,since recorded data is designed to be reproduced by irradiating a lightbeam from the protective film side, the substrate is not necessarilyrequired to be transparent. Since the light beam is irradiated from theprotective film side as mentioned above, the problem of birefringence ofthe substrate can be disregarded. Therefore, there is not any particularlimitation as to the material of the substrate as long as the materialis excellent in environmental resistance, heat resistance andworkability. For example, materials such as ABS resin, polyethyleneresin, polystyrene resin, etc. which are inexpensive as compared withthe materials employed conventionally can be employed.

FIG. 2 shows a graph illustrating a relationship between the filmthickness and reflectance of the reflective film of the opticalrecording medium shown in FIG. 1.

By the way, the reflective film in this example was formed using anAl-based alloy film, and the wavelength of the light beam to beirradiated was set to 650 nm. The graph of FIG. 2 shows that under thiscondition, when the film thickness of the reflective film 3 is set toabout 14 nm, the reflectance becomes 45%, while when the film thicknessof the reflective film 3 is set to about 40 nm, the reflectance becomesalmost saturated. The reflectance represented by the curve shown in thisgraph includes also a surface reflection from the surface of theprotective film 4. Accordingly, the reflectance at the surface of thereflective film can be obtained by subtracting the surface reflectionfrom the reflectance shown in this graph. Specifically, since thereflectance when the thickness of the reflective film is zerocorresponds to the surface reflection of the protective film 4, thereflectance of the reflective film can be obtained by subtracting thevalue (about 5%) of the reflectance when the thickness of the reflectivefilm is zero from the reflectance shown in this graph in the case of thecurve shown in FIG. 2.

FIG. 3 illustrates the dependency of the film thickness of a reflectivefilm on the light wavelength when the reflectance becomes 45% orsaturated. By the way, the reflective film in this example was formedusing an Al-based alloy film, and the wavelength of the light beam wasset to 400 to 800 nm.

In the graph shown in FIG. 3, the line “a” indicates a film thicknesswhere the reflectance becomes 45%, while the line “b” indicates a filmthickness where the reflectance becomes saturated. As shown in thisgraph, the dependency of the film thickness of the reflective film onthe light wavelength within the range of 400 to 800 nm is relativelysmall. Further, within this range of wavelength, the film thickness ofthe reflective film giving a reflectance of 45% was in the range of 13to 14 nm, while the film thickness of the reflective film giving a.saturated reflectance was about 40 nm.

Since the reflectance is set to 45 to 85% according to the specificationof DVD-ROM, the reflectance of the reflective film of the optical diskaccording to this invention is required to be 45% or more thereby makingit possible to secure the compatibility with DVD-ROM in the reproductionof data. The graph shown in FIG. 3 indicates that for the realization ofthis reflectance, the film thickness of the reflective film is requiredto set to 14 nm or more. Further, for the purpose of securing a fixedreflectance by suppressing the fluctuation in film thickness of thereflective film, it is preferable to set the reflective film to a filmthickness where the reflectance thereof is saturated. For this purpose,the film thickness of the reflective film should preferably be set to 40nm or more.

The reflective film consisting of the Al-based alloy film andmanufactured as mentioned above is minimal in light wavelengthdependency, so that it is possible to apply the reflective film withthis magnitude of film thickness to the reproduction of data using agreen or blue light beam in future.

EXAMPLE 2

FIG. 4 is a cross-sectional view schematically showing one example of anoptical disk according to this example. In the optical disk shown inFIG. 4, one of the surfaces of a disk substrate 1 having a thickness of1.2 mm is formed into a recording surface on which emboss pits 2 areformed in conformity with a recorded data. The surface of this recordingsurface provided with the emboss pits 2 is covered by a reflective film3, and then cover-coated with a first protective film 4 and a secondprotective film 18 in the mentioned order. In the case of the opticaldisk shown in FIG. 4, the surface of the second protective film 18 isthe light incident surface.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 4, a light beam 5 is converged by anobjective lens 6 and then enters through the second protective film 18.The light beam is then reflected by the reflective film 3, thusproducing a light reflection accompanying changes in light intensity,which are then detected as recorded data of the emboss pits.

The:disk substrate 1 having a thickness of 1.2 mm, the emboss pits 2,the reflective film 3 and the first protective film 4 all constitutingthe optical disk of this example can be constructed in the same manneras those of the aforementioned optical disk of Example 1.

The second protective film 18 to be placed on the surface of the firstprotective film 4 can be formed by making use of a conventionalultraviolet-curing resin for instance. Namely, an ultraviolet-curingresin for instance is at first coated on the surface of the firstprotective film 4 by means of a spin coating method thereby to form aresin film, and then irradiated with ultraviolet rays thereby to curethe resin film, thus forming the second protective film 18. Thethickness of this over-coating second protective film 18 may be in therange of from several microns to several millimeters in practical viewpoint. More preferably, the thickness of this over-coating secondprotective film 18 should be not more than 0.6 mm. Further, in view ofthe film thickness distribution of the ultraviolet-curing resin thatwill be obtained by means of a spin-coating method, the thickness ofthis over-coating second protective film 18 should preferably be in therange of 0.0001 to 0.1 mm in practical viewpoint. Additionally, thethickness of the second protective film 18 should preferably be suchthat does not optically interfere with the reflective film 3.

As for the material for the second protective film 18, it is notrestricted to ultraviolet-curing resins, but may be any material as longas it is capable of allowing a light beam for the reproduction of datato be transmitted therethrough and is stable environmentally andthermally. For example, the second protective film 18 may be constitutedby a dielectric material. More specifically, the second protective film18 may be formed by means of a vacuum deposition method or a sputteringmethod employing SiO₂, SiO, AlN, Al₂O₃, ZrO₂, TiO₂, Ta₂O₃, ZnS, Si, Geor a mixture thereof.

Further, this second protective film 18 is not limited to theaforementioned materials and to the aforementioned film-forming method,but may be made from any materials which are transparent to thewavelength of light to be employed. For example, a film-like or aplate-like transparent resin having a thickness ranging from 0.0001 mmto 0.6 mm may be employed. These resin film or plate may be placed onthe first protective film 4 at the occasion of spin-coating the firstprotective film 4, and then UV-irradiated and cured in a UV furnace,thus causing the resin film or plate to be adhered onto the firstprotective film 4.

However, the refractive index of the second protective film 18 and therefractive index of the first protective film 4 should be suitablyselected to meet the following relationship. Specifically, therefractive index n₂ of the second protective film 18 at the wavelengthof the light beam to be employed for the reproduction of data isrequired to be larger than or equivalent to the refractive index n₁ ofthe first protective film 4 at the wavelength of the light beam to beemployed for the reproduction of data. If the refractive index of thesecond protective film 18 is smaller than the refractive index of thefirst protective film 4 (n₁>n₂), the light reflection at the interfacebetween the second protective film 18 and the first protective film 4becomes larger, whereby the signal from the recording surface isdeteriorated and the light efficiency is caused to deteriorate.

Accordingly, in the case where the optical disk is constituted by a2-ply structure of protective films as shown in FIG. 4, the secondprotective film 18 is required to be formed of a transparent materialhaving a refractive index n₂ which is larger than or equivalent to therefractive index n₁ of the first protective film 4.

According to the optical disk of this example, since the firstprotective film 4 is covered by the second protective film 18, itbecomes possible not only to enhance the mechanical strength of thesurface of the disk, but also to prevent the surface of the disk frombeing damaged during the handling of the disk.

EXAMPLE 3

FIG. 5 is a cross-sectional view schematically showing one example of anoptical disk according to this example. In the optical disk shown inFIG. 4, one of the surfaces of a disk substrate 25 having a thickness of1.2 mn is provided with guiding grooves 26 to be used for the trackingof light beam 5. On this guiding grooves 26 are further formed areflective film 27, a first protective film 28 constituting a lowerprotective film, a recording film 29, and a second protective film 30constituting a upper protective film in the mentioned order. The surfaceof this second protective film 30 is further over-coated by a thirdprotective film 31. In the case of the optical disk shown in FIG. 5, thesurface of the third protective film 31 is the light incident surface.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 5, a light beam 5 is converged by anobjective lens 6 and then enters through the third protective film 31.The light beam is then reflected by the reflective film 3, thusproducing a light reflection accompanying changes in light intensity dueto the recording marks, the changes of which are then detected asrecorded data.

This disk substrate 25 can be constituted by a material which is stableand minimal in change with time. Examples of specific useful materialsare acrylic resin such as polymethylmethacrylate (PMMA), polycarbonateresin, epoxy resin, styrene resin, glass, a metal such as Al, an alloyand ceramics. The surface of the optical disk substrate 25 constructedwith any of these materials is then provided with groove tracks, landtracks, preformat marks, etc. depending on a recording format.

The reflective film 27 has not only the effect of optically enhancingthe optical changes of the recording film 29 to be formed via the lowerprotective film 28 on the reflective film 27 thereby to enhance thereproducing signal, but also the effect of cooling the recording film29. The reflective film 27 can be formed by depositing, by means ofvacuum deposition method or a sputtering method, a metallic materialsuch as Au, Al, Cu, Ni—Cr or an alloy containing any of these materialas a main component. The film thickness of the reflective film 27 may beseveral nanometers to several micrometers in practical viewpoint.

The recording film 29 can be constituted by a phase-changing materialwhose crystal structure is adapted to be changed by the condition ofirradiating a light beam. Examples of this phase-changing type materialare chalcogenide type amorphous semiconductor materials such as GeTetype, TeSe type, GeSbSe type, TeO_(x) type, InSe type and GeSbTe typeamorphous semiconductor materials and compound semiconductor materialsuch as InSb type, GaSb type and InSbTe type compound semiconductormaterials. The recording film 29 can be formed by means of a vacuumdeposition method or a sputtering method employing above materials. Thefilm thickness of the recording film 29 may be several nanometers toseveral micrometers in practical viewpoint.

The lower protective film 28 and the upper protective film 30 arelaminated with the recording film 29 being interposed therebetween,thereby functioning to prevent the recording film 29 from beingdispersed or holed due to the irradiation of a recording beam. The lowerprotective film 28 and the upper protective film 30 are also effectivein controlling the heat diffusion in the heating and cooling of therecording film 29 at the occasion of recording. These lower protectivefilm 28 and upper protective film 30 can be formed by means of a vacuumdeposition method or a sputtering method employing SiO₂, SiO, AlN,Al₂O₃, ZrO₂, TiO₂, Ta₂O₃, ZnS, Si, Ge or a mixture thereof. The filmthickness of these lower protective film 28 and upper protective film 30may be several nanometers to several micrometers in practical viewpoint.

The over-coating protective film (third protective film) 31 to be formedon the surface of this upper protective film (second protective film) 30is provided for the purpose of preventing the phase-changing opticaldisk from being contaminated during the handling of the optical disk,and can be normally formed using a ultraviolet-curing resin. Namely, anultraviolet-curing resin for instance is at first coated on the surfaceof the upper protective film 30 by means of a spin coating methodthereby to form a resin film, and then irradiated with ultraviolet raysthereby to cure the resin film, thus forming the over-coating protectivefilm 31.

The thickness of this over-coating protective film 31 may be in therange of from several microns to several millimeters in practical viewpoint. More preferably, the thickness of this over-coating protectivefilm 31 should be not more than 0.6 mm. On the other hand, the lowerlimit in thickness of the third protective film 31 should preferably be5λ/(4n) or more (wherein λ is a wavelength of the light beam; and n is arefractive index of the third protective film to a light having awavelength of λ). If the film thickness of the third protective film 31is less than 5λ/(4n), it becomes difficult to reproducing the recordeddata normally, since a multi-interfering effect of the light beam tendsto occur. Further, in view of the film thickness distribution of theultraviolet-curing resin that will be obtained by means of aspin-coating method, the thickness of this over-coating protective film31 should preferably be in the range of 0.0001 to 0.1 mm in practicalviewpoint. Additionally, the thickness of the over-coating protectivefilm 31 should preferably be such that does not optically interfere withthe reflective film 27.

In the foregoing explanation, a phase-changing recording mediumconstituted by a 4-ply structure has been explained as one example.However, each layer may be formed of a multi-layer depending on theperformance to be demanded.

For example, a fourth protective film may be formed on the protectivefilm 31, thereby making the surface of the optical disk into a 2-plyprotective film structure. This structure is effective in enhancing themechanical strength of the surface of the optical disk and in preventingthe optical disk from being damaged during the handling of the opticaldisk.

However, the refractive index of the fourth protective film and therefractive index of the third protective film 31 should be suitablyselected to meet the following relationship. Specifically, therefractive index n₄ of the fourth protective film at the wavelength ofthe light beam to be employed for the reproduction of data is requiredto be larger than or equivalent to the refractive index n₃ of the thirdprotective film 31 at the wavelength of the light beam to be employedfor the reproduction of data. If the refractive index of the fourthprotective film is smaller than the refractive index of the thirdprotective film 31 (n₃>n₄), the light reflection at the interfacebetween the fourth protective film and the third protective film 31becomes larger, whereby the signal from the recording surface isdeteriorated and the light efficiency is caused to deteriorate.

Accordingly, in the case where the optical disk is constituted by a2-ply structure of protective films, the fourth protective filmconstituting an outermost surface is required to be formed of atransparent material having a refractive index n₄ which is larger thanor equivalent to the refractive index n₃ of the third protective film31.

EXAMPLE 4

FIG. 6 is a cross-sectional view schematically showing one example of anoptical disk according to this example. In the optical disk shown inFIG. 6, both top and back surfaces of a disk substrate 13 having athickness of 1.2 mm are formed into a recording surface on which embosspits 2 are formed in conformity with a recorded data. Each surface ofthese recording surfaces provided with the emboss pits 2 are covered bya reflective film 3, and then cover-coated with a protective film 4. Inthe case of the optical disk shown in FIG. 6, the surfaces of thiscouple of the protective films 4 constitute individually the lightincident surfaces.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 6, a light beam 5 is converged by anobjective lens 6 and then enters through both of the protective films 4.The light beam is then reflected by the reflective film 3, thusproducing a light reflection accompanying changes in light intensity,which are then detected as recorded data of the emboss pits. Since therecording surface is formed on both surfaces of the optical diskaccording to this example, the reproduction of data can be effected fromboth surfaces, thereby making it possible to secure a recording capacitywhich is twice as large as that of the single recording surface.

FIG. 7 shows schematically one example of a mold to be employed forinjection-molding a disk substrate 13 to be employed for manufacturingthe optical disk of this example and having a thickness of 1.2 mm and arecording surface on both sides thereof.

If it is designated that one of the surface of the disk: is an Asurface, and the other surface is a B surface, a stamper 7a for the Asurface and a stamper 7b for the B surface are respectively manufacturedat first by making use of a conventional mastering. Then, a central hole8 is accurately formed at the center of the stamper 7a for the Asurface. Likewise, a central hole 14 is accurately formed at the centerof the stamper 7b for the B surface.

A projection 16 having a diameter corresponding to the central hole 8 ofthe stamper 7a for the A surface is formed at the central portion of theinner surface of the mold 9. Likewise, a projection 17 having a diametercorresponding to the central hole 14 of the stamper 7b for the B surfaceis formed at the central portion of the inner surface of the mold 15. Byproviding these projections 16 and 17, the central hole 8 of the stamper7a for the A surface can be accurately aligned with the central hole 14of the stamper 7b for the B surface.

Then, the stamper 7a for the A surface is mounted on the inner surfaceof the mold 9 for injection molding in such a manner that the recordingsurface of the stamper 7a is directed inside and the projection 16 ofthe mold 9 is inserted into the central hole 8 of the stamper 7a for theA surface. Likewise, the stamper 7b for the B surface is mounted on theinner surface of the mold 15 disposed to face the mold 9 in such amanner that the recording surface of the stamper 7b is directed insideand the projection 17 of the mold 15 is inserted into the central hole14 of the stamper 7b for the B surface. Under the condition where theseprojections 16 and 17 are inserted into the central holes 8 and 14 ofthe stampers, respectively, the central hole 8 of the stamper 7a for theA surface is adjusted to accurately align with the central hole 14 ofthe stamper 7b for the B surface.

Then, under the condition where the recording surface of the stamper 7afor the A surface is disposed to face the recording surface of thestamper 7b for the B surface, a heated and molten resin is introducedfrom the resin inlet port 12 into the space between these recordingsurfaces. Thereafter, either the stamper 7a for the A surface or thestamper 7b for the B surface is moved forward thereby to adjust theinterval between the stamper 7a for the A surface and the stamper 7b forthe B surface to a predetermined size. Specifically, the intervalbetween the stamper 7a for the A surface and the stamper 7b for the Bsurface is set to such that the thickness of the substrate after thecooling or curing thereof becomes 1.2 mm.

As a result of the aforementioned process, a disk substrate 13 having athickness of 1.2 mm and provided with recording surfaces on both sidesthereof can be molded by a single injection molding step.

The deposition of the reflective film 3 on these recording surfaces canbe performed by depositing a material for the reflection film by meansof a vacuum deposition method or a sputtering method. More specifically,an evaporation source or a sputtering target material is placed on bothsides of the disk, thereby positioning a pair of evaporation sources orsputtering target materials so as to face each other. Then, theevaporation sources or sputtering target materials are allowed toevaporate or sputter, thereby simultaneously depositing a pair ofreflective films on both recording surfaces. Alternatively, it is alsopossible to mask one of the recording surfaces with a masking materialand to perform the aforementioned deposition, and thereafter, the sameprocedures are repeated, thereby successively performing the depositionof the reflective film one by one.

The over-coating of the protective film 4 can be performed by a processwherein a masking material is applied at first to one of the reflectivefilm 3, a UV-curing resin is spin-coated on this one of the reflectivefilm 3, a UV-curing resin is then spin-coated on the other one of thereflective film 3 in the same manner, then the resultant resin films arecured in a UV furnace, thus forming the protective film 4.Alternatively, the over-coating of the protective film 4 can beperformed simultaneously on both reflective films 3. In this case, thedisk should be rotatably supported on a suitable tool and then aUV-curing resin is applied through spin-coating to both sides of thedisk thereby to form a resin film on the reflective films 3. Theresultant resin films on the reflective films 3 are then allowed to passthrough a pair of facing UV lamps provided in a UV furnace thereby tocure the resin films. By the utilization of this process, theover-coating of protective films 4 can be performed simultaneously onboth reflective films 3.

EXAMPLE 5

FIG. 8 is a cross-sectional view schematically showing one example of anoptical disk according to this example. In the optical disk shown inFIG. 8, both top and back surfaces of a disk substrate 13 having athickness of 1.2 mm are formed into a recording surface on which embosspits 2 are formed in conformity with a recorded data. The surfaces ofthese recording surfaces provided with the emboss pits 2 are coveredrespectively by a reflective film 3, by a first protective film 4 and asecond protective film 18. In the case of the optical disk shown in FIG.8, the surfaces of this couple of the second protective films 18constitute individually the light incident surface.

The structure of the optical disk according to this example is the sameas that of Example 2 except that the recording surface and other filmsare formed on both surface of the disk substrate. Therefore, the disksubstrate 13 having a thickness of 1.2 mm and constituting the opticaldisk of this example can be constructed in the same manner as explainedin Example 4, and other components such as emboss pits 2, reflectivefilm 3, first protective film 4 and second protective film 18 can beconstructed in the same manner as explained in Example 1.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 8, a pair of light beams 5 and 20 areconverged by objective lens 6 and 21, respectively, and then allowed toenter through the second protective films 18 disposed on both sides ofthe disk. The light beams are then reflected by the reflective film 3,thus producing a light reflection accompanying changes in lightintensity, which are then detected as recorded data of the emboss pits.Since the recording surface is formed on both surfaces of the opticaldisk according to this example, the reproduction of data can be effectedfrom both surfaces, thereby making it possible to secure a recordingcapacity which is twice as large as that of the single recordingsurface.

The second protective film 18 to be placed on the surface of the firstprotective film 4 can be formed by making use of a conventionalultraviolet-curing resin for instance. Namely, an ultraviolet-curingresin for instance is at first coated on the surface of the firstprotective film 4 by means of a spin coating method thereby to form aresin film, and then irradiated with ultraviolet rays thereby to curethe resin film, thus forming the second protective film 18.

This couple of the second protective film 18 can be formed one afteranother or concurrently on both surfaces of the disk. Namely, if thesecond protective film 18 is to be formed one after another on bothsurfaces of the disk, a masking material is applied at first onto one ofthe first protective film, and then a UV-curing resin is spin-coated onthis one of the first protective film. Then, the same procedures asmentioned above are repeated on the other one of the first protectivefilm, thereby forming a couple of resin films. Thereafter, the resultantresin films are cured in a UV furnace, thus easily forming the secondprotective film 18. On the other hand, if a couple of the secondprotective films 18 are to be formed simultaneously on both firstprotective films 4, the following procedures can be employed. In thiscase, the disk should be rotatably supported on a suitable tool and thena UV-curing resin is applied through spin-coating to both sides of thedisk thereby to form a resin film on both first protective films 4. Theresultant resin films on both first protective films 4 are then allowedto pass through a pair of facing UV lamps provided in a UV furnacethereby to cure the resin films. By the utilization of this process, theover-coating of the second protective film 18 can be performedsimultaneously on both first protective films 4.

The thickness of this over-coating second protective film 18 may be inthe range of from several microns to several millimeters in practicalview point. More preferably, the thickness of this over-coating secondprotective film 18 should be not more than 0.6 mm. Further, in view ofthe film thickness distribution of the ultraviolet-curing resin thatwill be obtained by means of a spin-coating method, the thickness ofthis over-coating second protective film 18 should preferably be in therange of 0.0001 to 0.1 mm in practical viewpoint. Additionally, thethickness of the second protective film 18 should preferably be suchthat does not optically interfere with the reflective film 3.

As for the material for the second protective film 18, it is notrestricted to ultraviolet-curing resins, but may be any material as longas it is capable of allowing a light beam for the reproduction of datato be transmitted therethrough and is stable environmentally andthermally. For example, the second protective film 18 may be constitutedby a dielectric material. More specifically, the second protective film18 may be formed by means of a vacuum deposition method or a sputteringmethod employing SiO₂, SiO, AlN, Al₂O₃, ZrO₂, TiO₂, Ta₂O₃, ZnS, Si, Geor a mixture thereof.

If the second protective film 18 is to be simultaneously formed on bothof the first protective films 4 by making use of these materials, anevaporation source or a sputtering target material is placed on bothsides of the disk, thereby positioning a pair of evaporation sources orsputtering target materials so as to face each other. Then, theevaporation sources or sputtering target materials are allowed toevaporate or sputter, thereby simultaneously depositing a pair of thesecond protective films 18 on both of the first protective films 4,respectively. Alternatively, it is also possible to mask one of thefirst protective films 4 with a masking material and to perform thedeposition of the second protective films 18, and thereafter the sameprocedures as mentioned above are repeated, thereby successivelyperforming the deposition of the reflective film one by one.

Further, this second protective film 18 is not limited to theaforementioned materials and to the aforementioned film-forming method,but may be made from any materials which are transparent to thewavelength of light to be employed. For example, a film-like or aplate-like transparent resin having a thickness ranging from 0.0001 mmto 0.6 mm may be employed. These resin film or plate may be placed onthe first protective film 4 at the occasion of spin-coating the firstprotective film 4, and then UV-irradiated and cured in a UV furnace,thus causing the resin film or plate to be adhered onto the firstprotective film 4.

If it is desired to form the second protective film 18 by adhering theaforementioned resin film onto the first protective film 4, a maskingmaterial is applied at first onto one of the surfaces of the reflectivefilm, and then a UV-curing resin is coated on the other surface of thereflective film. Then, the same procedures as mentioned above arerepeated on the other surface of the reflective film, thereby forming acouple of resin films for constituting the first protective films 4.Thereafter, a material for the second protective film 18 is spin-coatedon these resin films and then cured in a UV furnace, thereby easilyforming the second protective films 18. The second protective films 18may be formed concurrently. In this case, the disk should be rotatablysupported on a suitable tool and then a UV-curing resin is appliedthrough spin-coating to both sides of the disk thereby to form a resinfilm on both first protective films 4. The resultant resin films on bothfirst protective films 4 are then allowed to pass through a pair offacing UV lamps provided in a UV furnace thereby to cure the resinfilms, thus concurrently forming the second protective films 18.

If the thickness of the protective film 18 formed on both sides of thedisk is set to correspond with the operating distance of the light beam,and at the same time, if this couple of protective films disposed onboth top and back surfaces of the disk are made identical in thicknessthereof with each other, the recorded data on both surfaces of the diskcan be reproduced by making use of an optical head having an identicaloperation distance.

However, because of the same reason as explained in the aforementionedExample 2, when an optical disk is constituted by a 2-ply laminationstructure of protective film as shown in FIG. 8, the second protectivefilm 18 should be formed using a transparent material having arefractive index n₂ which is larger than or equivalent to the refractiveindex n₁ of the first protective film 4.

The optical disk according to this example is constructed such that thesecond protective film 18 is further laminated on each of the firstprotective films 4 formed on both sides of the disk. In the case ofExample 3, the recording. surface is formed only one surface of thedisk, i.e. a single recording type. Whereas, in this example, therecording surface is formed also on the other side of the disk, thusmaking it into a double recording type.

Since the optical disk is made into a double recording type, it ispossible to secure a recording capacity which is twice as large as thatof the optical disk of single recording type.

According to the optical disk of this example, since both of the firstprotective films 4 are respectively covered by the second protectivefilm 18, it becomes possible not only to enhance the mechanical strengthof both surfaces of the disk, but also to prevent both surfaces of thedisk from being damaged during the handling of the disk.

EXAMPLE 6

The film thickness of the couple of second protective films 18 formed onboth sides of the disk shown in Example 5 may be modified to becomedifferent from each other.

For example, in the optical disk shown in FIG. 8, the film thickness ofone of the second protective films 18 is set to in the range of 0.0001to 0.6 mm, while the film thickness of the other second protective film18 is set to 0.6 mm. As a result, the following advantages can beobtained. Namely, the surface on one of the protective films 18 that isthinner in film thickness can be employed as a data-reproducing surfacefor an optical head having a shorter operating distance, while thesurface on the other protective film 18 that is larger in film thicknesscan be employed as a data-reproducing surface for an optical head formedin conformity with the conventional DVD specification.

EXAMPLE 7

The thickness of the entire disk shown in Example 6 may be set to 1.2 mmby further adjusting the thickness of the substrate 13.

For example, when the film thickness of one of the second protectivefilms 18 is set to 0.1 mm, while the film thickness of the other secondprotective film 18 is set to 0.6 mm in the optical disk shown in FIG. 8,the thickness of the entire disk can be made into 1.2 mm by setting thethickness of the disk substrate 13 to 0.5 mm. As a result, the thicknessof the optical disk can be made equivalent to that of the conventionalCD and DVD, so that the compatibility in the handling of disk can besecured.

EXAMPLE 8

FIG. 9 is a cross-sectional view schematically showing one example of anoptical disk according to this example.

In the optical disk shown in FIG. 9, both top and back surfaces of adisk substrate 34 having a thickness of 1.2 mm are provided with guidinggrooves 26 for the tracking of the light beams 5 and 32. The surface ofthe guiding grooves 26 is covered by a sequence of films including, inthe mentioning order, a reflective film 27, a first protective film 28as an lower protective film, a recording film 29, and a secondprotective film 30 as an upper protective film. Further, a thirdprotective film 31 is over-coated on the second protective film 30. Inthe case of the optical disk shown in FIG. 9, the surfaces of thiscouple of the third protective film 31 constitute individually the lightincident surface.

The structure of the optical disk according to this example is the sameas that of Example 3 except that the guiding grooves for tracking andother films are formed on both surface of the disk substrate. Therefore,the material of the disk substrate for constituting the optical disk ofthis example and the properties of other films are the same as those ofExample 3. The formation of the reflective film, recording film and eachprotective film on both surfaces of the disk substrate can be performedin the same manner as explained in Example 5.

The:readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 9, a pair of light beams 5 and 32 areconverged by objective lens 6 and 33, respectively, and then allowed toenter through the third protective films 31 disposed on both sides ofthe disk. The light beams are then reflected from the recording film 29according to the recording marks, thus producing a light reflectionaccompanying changes in light intensity, which are then detected asrecorded data. Since the recording surface is formed on both surfaces ofthe optical disk according to this example, the reproduction of data canbe effected from both surfaces, thereby making it possible to secure arecording capacity which is twice as large as that of the singlerecording surface.

The over-coating protective film (the third protective film) 31 to beformed on the upper protective film (the second protective film) 30 canbe formed by making use of the same materials as employed in theaforementioned Example 3. The film thickness of the over-coatingprotective film 31 may be the same as that of Example 3.

In the foregoing explanation, a phase-changing recording mediumconstituted by a 4-ply structure has been explained as one example.However, each layer may be formed of a multi-layer depending on theperformance to be demanded.

For example, a fourth protective film may be formed on the protectivefilm 31, thereby making the surface of the optical disk into a 2-plyprotective film structure. This structure is effective in enhancing themechanical strength of the surface of the optical disk and in preventingthe optical disk from being damaged during the handling of the opticaldisk.

However, because of the same reason as explained in the aforementionedExample 3, when the surface of the optical disk is constituted by a2-ply lamination structure of protective film, the fourth protectivefilm to be formed as an outermost surface should be formed using atransparent material having a refractive index n₄ which is larger thanor equivalent to the refractive index n₃ of the third protective film31.

EXAMPLE 9

A recessed/projected surface constituting a recording emboss or atracking groove is formed inside the protective film constituting anoutermost surface, or inside the disk in the optical disks explained inthe aforementioned Examples 1 to 8. In this case, a specific example ofthe protective film constituting an outermost surface may be the firstprotective film 4 as shown in FIGS. 1 and 6, the second protective film18 as shown in FIGS. 4 and 8, or the third protective film 31 as shownin FIGS. 5 and 9.

In this case, these protective films may be formed as a thick film onthe outside of the disk. As a result, it becomes possible to flatten thesurface without giving an effect to the recessed/projected surfaceformed on the inside of the disk.

EXAMPLE 10

This example illustrates a large capacity optical disk provided with amultiple information recording layer. In particular, this optical diskis a 2-ply optical disk which is designed to make it possible to readout information of a couple of information recording layers from onesurface of a reproducing or recording optical disk. Namely, it is nowpossible, according to the 2-ply optical disk of this example, torealize a 2-ply optical disk as an optical disk that can be reproducedor recorded from the top surface, and at the same time, to improve thereadout S/N of the inner recording layer, and to ensure the readout S/Nof the outer recording layer, thus enabling the 2-ply optical disk to bepractically employed.

FIG. 10 shows a cross-sectional view schematically illustrating oneexample of the optical disk of this example.

The optical disk shown herein includes at least an over-coat film orsurface substrate 51 provided with a light incident surface and anemboss pit surface, a first reflective film 52, a transparent adhesionlayer (a spacer layer) 53, a second reflective film 54 and a supportingsubstrate (a core substrate) 55 provided with an emboss pit surface anda substrate surface. The dimension of this disk are; 120 mm in outerdiameter, 15 mm in inner diameter (a diameter of the central hole) and1.2 mm in thickness, wherein the thickness of the core substrate 55 isabout 1.1 mm, and the thickness of the surface substrate 51 is smallerthan that of the core substrate 55, i.e. about 0.001 to 0.1 mm.

A first recording surface is constituted by the emboss pit surface ofthe surface substrate 51 and by the first reflection film 52, while thesecond recording surface is constituted by the emboss pit surface of thesupporting substrate 55 and by the second reflection film 54.

The readout of the recorded data stored in the disk can be performed asfollows. Namely, as shown in FIG. 10, a light beam 70 is converged by anobjective lens 71, then enters through the surface substrate 51, andconverged at the first recording surface or the second recordingsurface. The light beam is then reflected by each of these recordingsurfaces, thus producing a light reflection accompanying changes inlight intensity, which are then detected as recorded data of the embosspits.

According to the conventional optical disk, the emboss data are designedto be read by irradiating a light beam from the core substrate side. Asa result, the securing of the tilt margin is restricted depending on thethickness of the substrate. Whereas, according to the optical disk shownin FIG. 10, the data thereof is reproduced by irradiating a light beamthrough the surface substrate (or a covering substrate) 51. In thiscase, since the thickness of the covering substrate 51 is smaller thanthe thickness of the core substrate 55, the tilt margin is no morerestricted by the thickness of the substrate, thus making it possible tofurther increase the recording density.

The core substrate 55 having a thickness of 1.1 mm can be manufacturedby means of an injection molding method which is commonly employed forthe manufacture of the conventional CD and DVD. For example, a masterplaten in which information is stored in advance is mounted on one ofthe dies of an injection molding machine, and then an injection moldingis performed after adjusting the space between a couple of dies in sucha way that the thickness of the substrate after the molding thereofbecomes 1.1 mm, thus manufacturing the core substrate 55 having athickness of 1.1 mm.

On the other hand, the covering substrate 51 is very thin in thickness,i.e. about 0.1 mm in thickness. Therefore, the injection molding methodwhich is commonly employed for the manufacture of the conventional CDand DVD can hardly be applied as it is to the manufacture of thecovering substrate 51. Accordingly, according to this invention, thefollowing method is employed for the manufacture of the coveringsubstrate 51.

The injection molding method of the covering substrate 51 according tothis invention will be explained with reference to FIG. 11.

First of all, a flat supporting substrate 56 is mounted on one innerside of a molding die 59, and then, a releasing agent 57 is coated onthe flat supporting substrate 56. On the other hand, a master platen 58in which a data has been stored in advance is mounted on the oppositeinner side of the molding die 59 so as to rendering the master platen 58to face the flat supporting substrate 56.

Then, the cavity space of the molding die is adjusted to such that thethickness of the covering substrate 51 becomes 0.1 mm after the molding.Thereafter, a heated and molten resin is introduced from the resin inletport into the space between the stamper 58 of the molding die 59 and thesupporting substrate 56. Then, either the surface of the stamper and thesurface of the supporting substrate are pushed forward thereby to setthe interval thereof to a predetermined size. Specifically, the intervalbetween the surface of the stamper and the surface of the supportingsubstrate is set to such that the thickness of the covering substrate 51after the cooling or curing thereof becomes 0.1 mm.

As a result of this injection molding, a covering substrate: 51 having athickness of 0.1 mm and bonded via the releasing agent 57 to thesupporting substrate 56 can be formed.

The surfaces of both covering substrate 51 and core substrate 55 aredeposited thereon with a predetermined thickness of reflective film bymeans of a sputtering method for instance. The deposition of thereflective film on the covering substrate 51 can be performed bymounting the covering substrate 51 on a sputtering apparatus whilekeeping the covering substrate 51 attached through the releasing agent57 to the supporting substrate 56. Since the covering substrate 51 iskept sustained on the supporting substrate 56, the covering substrate 51can be prevented from being warped during the sputtering and at the sametime, the handling of the covering substrate 51 can be facilitated.

Both covering substrate 51 and core substrate 55 coated respectivelywith a reflective film are bonded together to each other according tothe following process. Namely, FIGS. 12A to 12D illustrate the processof bonding the covering substrate 51 to the core substrate 55 after thedeposition of the reflective film.

First of all, as shown in FIG. 12A, the alignment pin 60 of a spin table61 is inserted into the central hole of the core substrate 55. In thiscase, the core substrate: 55 is arranged with the recording surfacethereof being faced upward. After the core substrate 55 is closelyattached onto the spin table 61, a suitable amount of aultraviolet-curing adhesive 62 of low viscosity is coated on the coresubstrate 55.

After the coating of this adhesive, the central hole of the coveringsubstrate 51 bonded to the supporting substrate 55 is fitted with thealignment pin 60 of the spin table 61 as shown in FIG. 12A. In thiscase, the covering substrate 51 is arranged with the recording surfacethereof being faced downward. The covering substrate 51 is adhered withthe core substrate 55 in such a manner that the adhesive 62 coated onthe core substrate 55 is spread out by the covering substrate 51.

Then, as shown in FIG. 12C, after the covering substrate 51 is adheredwith the core substrate 55, the spin table is caused to rotate at a highspeed. The superfluous portion of the adhesive interposed between thecovering substrate 51 and the core substrate 55 is splashed out due tothe centrifugal force of the high speed rotation, thereby making itpossible to form a uniform foamless adhesive layer 53 having apredetermined thickness between these substrates 51 and 55, thusobtaining a laminated optical disk.

Finally, as shown in FIG. 12D, ultraviolet rays are irradiated from anultraviolet lamp 63 onto the optical disk. As a result, the coveringsubstrate 51 and the core substrate 55 are completely integrated.

FIG. 13 illustrates a cross-sectional view of the laminated structure ofthe optical disk. As shown in FIG. 13, the supporting substrate 56 isbonded via the releasing agent layer 57 to the surface of the coveringsubstrate 51. Since the adhesive strength of the releasing agent layer57 is suitably weakened, the releasing agent layer 57 can be easilyseparated together with the supporting substrate 56 from the coveringsubstrate 51 when the supporting substrate 56 is pulled apart from thecovering substrate 51. As a result, the optical disk having a structureas shown in FIG. 10 can be obtained.

The surface of the covering substrate 51, i.e. the surface roughness “R”of the light incident surface should desirably be selected to meet arelationship represented by the following formula (1) even in the caseof the optical disk of this example:R≦λ/(8n)   (1)

-   -   wherein λ is a wavelength of the light beam; and n is a        refractive index of the covering substrate to a light having the        wavelength λ.

It becomes possible, by limiting the surface roughness of the lightincident surface as described above thereby ensuring the flatness of thelight incident surface, to minimize the light-diffracting phenomenon atthe light incident surface. Additionally, if the surface of the opticaldisk is flat as defined above, dust can be hardly attached to thesurface of the optical disk, thus improving a dust adhesion preventiveeffect of the optical disk. Moreover, if the surface of the optical diskis flat as defined above, it is also possible to obtain the effect thatthe optical disk can be prevented from being contacted with theobjective lens. By the way, more preferable surface roughness of thelight incident surface is not higher than a half of λ/(8n), i.e. nothigher than λ/(16n)

It is possible, even in the case of the optical disk shown in FIG. 10,to employ a phase-changing type recording film. Specifically, aphase-changing type recording film composed of any of such materials asexplained in the aforementioned Example 3 may be substituted for thereflective film. When the optical disk is constructed in this manner, arewritable optical disk can be obtained.

EXAMPLE 11

According to the conventional optical disk, a recorded data is designedto be reproduced by irradiating a light beam from the substrate side, sothat the substrate is required to be transparent to the wavelength of alight beam to be employed. By contrast, according to this invention,since a recorded data is reproduced by irradiating a light beam from thecovering substrate side, the core substrate is not necessarily requiredto be transparent. Additionally, the birefringence index of the coresubstrate can be disregarded in this invention.

Therefore, as a material for the core substrate of this invention, anykinds of material can be employed as long as they are excellent inenvironment resistance, heat resistance and workability. For example,materials such as ABS resin, polyethylene resin, polystyrene resin, etc.which are inexpensive as compared with the materials employedconventionally can be employed.

EXAMPLE 12

The reflectance of the first recording surface as well as of the secondrecording surface can be determined respectively by the reflectance andtransmittance of the first reflective layer 52 and second reflectivelayer 54.

FIG. 14 shows the reflectance and transmittance of each recordingsurface.

First of all, the reflectance and transmittance of the first recordingsurface are designated as R₁ and T₁, respectively, while the reflectanceand transmittance of the second recording surface are designated as R₂and T₂, respectively. When a light beam L₀ is irradiated from the firstrecording surface side, the reflected light L₁ of the light beam thathas been focused on the first recording surface can be represented bythe following formula (6).L₁=L₀×R₁/100   (6)

On the other hand, the transmitted light LT can be represented by thefollowing formula (7).LT=L₀×T₁/100   (7)

Further, the reflected light L₂ of the light beam that has been focusedon the second recording surface can be represented by the followingformula (8).

$\begin{matrix}\begin{matrix}{L_{2} = {\left( {{LT} \times {R_{2}/100}} \right) \times {T_{1}/100}}} \\{= {L_{0} \times \left( {R_{2}/100} \right) \times \left( {T_{1}/100} \right)^{2}}}\end{matrix} & (8)\end{matrix}$

In the case of the focused light to be emitted from the light head, ifthe interval between the first recording surface and the secondrecording surface is set away more or less from the focused point, thelight beam is caused to scatter at the regions other than the focusedrecording surface. Therefore, the leaking of the light beam would be sominimal that can be practically disregarded.

In view of reproducing data from each recording surface, the substantialreflectance of laser beam on every recording surface should desirably beset to a large value. Specifically, the ratio L₂ of a laser beam that isreflected from the lower second recording surface and passed through theupper first recording surface thereby to return to the outside inrelative to the external incident laser beam should preferably be about15 to 45%. Likewise, the ratio L₁ of a laser beam that is reflected fromthe upper first recording surface and returned to the outside inrelative to the external incident laser beam should preferably be about15 to 45%.

This means that the servo-property (servo-gain) of the data readoutoptical system of an apparatus for reproducing a data from a multipleinformation recording medium (for example, 2-ply optical disk) may bethe same irrespective of the position of the recording surface, i.e.irrespective of the upper first recording surface or the lower secondrecording surface. Namely, the switching of the servo-property of thedata readout optical system at the occasion of changing the readoutposition from the upper first recording surface to the lower secondrecording surface (and vice versa) is no more required. Accordingly, itis possible to avoid a temporary interruption (or the generation of atemporary operationally unstable state) of the reproducing operation atthe occasion of switching the readout layers. Since the switching of theservo-property of the data readout optical system is no more required,it is possible to expect various advantages that the operation of thereproducing apparatus can be further stabilized and that the cost formanufacturing the reproducing apparatus can be saved.

FIGS. 15 to 19 show graphs illustrating a relationship between the filmthickness of reflective film and the reflectance with respect to variousmaterials useful for the reflective film. The reflectance in this caseis a value in relative to a light beam having a wavelength of 400 nm.The film thickness and reflectance of the reflective film werecalculated under the condition wherein materials for each reflectivefilm were coated on the substrate thereby to form a film on which a UVresin was subsequently over-coated. By the way, the transmittance wasseparately calculated.

FIG. 15 illustrates a case wherein SiC was employed as a material forthe reflective film. The reflectance was 27% and the transmittance was48% as the film thickness was 30 nm. Since these values are acceptable,the SiC film can be employed as a reflective layer of the firstrecording surface.

FIG. 16 illustrates a case wherein Si was employed as a material for thereflective film. The reflectance was 40% and the transmittance was 20%as the film thickness was 20 nm. Since this Si film is large inabsorbency, it can be hardly employed as a reflective film of the firstrecording surface. However, if any other material exhibiting a hightransmittance is employed as a reflective layer of the first recordingsurface, this Si film can be employed as a reflective film of the secondrecording surface.

FIG. 17 illustrates a case wherein Au was employed as a material for thereflective film. The reflectance was 32% and the transmittance was 11%as the film thickness was 20 nm. Since this Au film is large inabsorbency, it can be hardly employed as a reflective film of the firstrecording surface. However, if any other material exhibiting a hightransmittance is employed as a reflective layer of the first recordingsurface, this Au film can be employed as a reflective film of the secondrecording surface.

FIG. 18 illustrates a case wherein SiO₂ was employed as a material forthe reflective film. As shown in this graph, the SiO₂ film issubstantially transparent to the light beam of 400 nm in wavelength, sothat the SiO₂ film can be hardly employed as a reflective film.

FIG. 19 illustrates a case wherein AlMo was employed as a material forthe reflective film. The reflectance was 70% as the film thickness was50 nm or more. The transmittance thereof is substantially zero.Therefore, the AlMo film is suited for use as a reflective film of thesecond recording surface.

In the case of this AlMo film, the reflectance was 32% and thetransmittance was 11% as the film thickness was 40 nm. Since this AlMofilm is large in absorbency, it can be hardly employed as a reflectivefilm of the first recording surface. However, if any other materialexhibiting a high transmittance is employed as a reflective layer of thefirst recording surface, this AlMo film can be employed as a reflectivefilm of the second recording surface.

For example, when an SiC film having a film thickness of 30 nm is formedas a reflective film of the first recording surface, and an AlMo filmhaving a film thickness of 50 nm is formed as a reflective film of thesecond recording surface, the reflectance L₁ of the first recordingsurface would become 27% and the reflectance L₂ of the second recordingsurface would become 16% when they are calculated according to theaforementioned formulas (6) and (8).

By the way, it is also possible to balance the reflectance of eachrecording surface by adjusting the film thickness of the SiC film to beformed on the surface of the first recording surface. For example, whenthe film thickness of this SiC film is set to 20 nm, the reflectance L₁of the first recording surface can be controlled to 20%, while thereflectance L₂ of the. second recording surface can be controlled to36%. Further, when the film thickness of this SiC film is set to 25 nm,the reflectance L₁ of the first recording surface can be controlled to23%, while the reflectance L₂ of the second recording surface can becontrolled to 23%.

EXAMPLE 13

FIG. 20A illustrates a disk structure where the covering layer of thesurface of the disk was formed using a UV over-coat film. It iseffective, in view of preventing the disk from being damaged during thehandling thereof, to form a recessed or projected portion 64 and 65 onthe region inside or outside the recording region. The provision of sucha recessed or projected portion can be performed by preliminarilyforming such a recessed or projected portion 64 and 65 on the surface ofthe supporting substrate 55 in prior to the formation of a UV over-coat.It is possible to form such a recessed or projected portion 64 and 65 onthe surface of the supporting substrate 55 by making use of a moldingdie or stamper provided with a corresponding recessed or projectedportion at the occasion of injection-molding the supporting substrate55.

The depth or height of the recessed or projected portion to be formed onthe surface of the supporting substrate 55 may be preferably about 0.3mm with the width thereof being about 0.2 to 10 mm in practicalviewpoint.

By the way, in view of preventing any bad influence from being given tothe tilt property at the occasion of clamping the disk, the over-coatfilm should not be applied to the clamping portion inside of the disk.

EXAMPLE 14

FIG. 20B illustrates a disk structure where the covering layer of thesurface of the disk was formed using a protective film having athickness of 0.1 mm. It is effective, in view of preventing the diskfrom being damaged during the handling thereof, to form a recessed orprojected portion 66 and 67 on the region inside or outside therecording region. The provision of such a recessed or projected portioncan be performed by making use of the supporting substrate 55 having aflat surface or a surface provided only recessed portions. It ispossible to form such a recessed or projected portion on the surface ofthe supporting substrate 55 by making use of a molding die or stamperprovided with a corresponding recessed or projected: portion at theoccasion of injection-molding the protective substrate 51.

The depth or height of the recessed or projected portion to be formed onthe surface of the supporting substrate 55 may be preferably about 0.3mm or less with the width thereof being about 0.2 to 10 mm in practicalviewpoint.

In this case, the protective film 51 is formed also at the clampingportion inside of the disk. This is because, the alignment between thesupporting substrate 55 and the protective substrate 51 is performed atthe spindle hole. Since the thickness of the protective substrate isrelatively uniform, the existence of the protective substrate at theclamping portion would not give a bad influence to the tilt property.

EXAMPLE 15

Next, the thickness of the adhesive layer 53 of the 2-ply disk will beexplained with reference to FIG. 21. FIG. 21 shows a relationshipbetween the wavelength of light beam for recording and reproducing adata and the intensity of reflected beam.

If the refraction index of the adhesive layer 53 is designated as “n”,the distance dad of the adhesive layer can be expressed by the followingformula (9).d_(ad)=λ₁×λ₂/2n(λ₁−λ₂)   (9)

-   -   wherein λ₁ and λ₂ are neighboring wavelengths under which        condition the intensity of reflected light becomes the maximum.

When. the wavelength for recording/reproducing is 650 nm, the distanced_(ad) of the adhesive layer is set to about 40 μm. At this occasion,the intensity of reflected light becomes the minimum.

Since the distance d_(ad) of the adhesive layer 53 is proportional tothe recording/reproducing wavelength, if the recording/reproducingwavelength is assumed as being 400 nm, the distance of the adhesivelayer which renders the intensity of reflected light to become theminimum is about 25 μm.d_(ad)=40×400/650=about 25 μm   (9)

Therefore, when the recording or reproducing wavelength is 400 nm, thedistance of the adhesive layer should preferably be set to about 25 μm,or practically in the range of 20 to 30 μm if the non-uniformity in themanufacture thereof is taken into account.

As explained above, it is possible according to this invention toprovide an information recording medium which is capable of securing asufficient tilt margin and a sufficient mechanical strength even if therecording density is further increased. Further, this invention providesa method of manufacturing a resinous substrate which is suited for usein the manufacture of such an information recording medium.

Since a substrate which is provided on both surfaces thereof with arecording surface can be easily manufactured according to the method ofthis invention and hence, the bonding of a couple of substrates can bedispensed with, the manufacturing process of the optical disk can beextremely simplified. Further, since an adhesive for the bonding of thesubstrates is no more required to be employed, it is possible to cheaplymanufacture an optical disk of large capacity.

Additionally, it is possible according to the information recordingmedium of this invention to secure a sufficient mechanical strength ofthe disk even if the surface protective film is made thinner than 0.6 mmwhich has been conventionally considered as a lower limit. Therefore,this invention would be very valuable in industrial viewpoint.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as

1. An information recording medium, which comprises: a supportingsubstrate; a first reflective layer formed on said supporting substrate;a spacer layer formed on said first reflective layer; a secondreflective layer formed on said spacer layer; and a covering layerformed on said second reflective layer and having a light incidentsurface; wherein a light beam can enter through said light incidentsurface and be reflected to produce a reflected light beam, a recordedinformation being reproduced based on changes in light intensity of thereflected light beam.
 2. The information recording medium according toclaim 1, wherein the thickness of said covering layer is in the range of0.0001 to 0.6 mm.
 3. The information recording medium according to claim2, wherein the thickness of said spacer layer is 30 μm or less.
 4. Theinformation recording medium according to claim 2, wherein the thicknessof said spacer layer is 20 μm or more.
 5. The information recordingmedium according to claim 2, wherein the reflectance of at least one ofsaid first reflective layer and said second reflective layer is 15% ormore.
 6. The information recording medium according to claim 2, whereinthe reflectance of at least one of said first reflective layer and saidsecond reflective layer is 45% or less.
 7. An information recordingmedium, which comprises: a supporting substrate; a first recording layerformed on said supporting substrate; a spacer layer formed on said firstrecording layer; a second recording layer formed on said spacer layer;and a covering layer formed on said second recording layer and having alight incident surface; wherein a light beam can enter through saidlight incident surface and be reflected to produce a reflected lightbeam, a recorded information being reproduced based on changes in lightintensity of the reflected light beam.
 8. The information recordingmedium according to claim 7, wherein the thickness of said coveringlayer is in the range of 0.0001 to 0.6 mm.
 9. The information recordingmedium according to claim 8, wherein the thickness of said spacer layeris 30 μm or less.
 10. The information recording medium according toclaim 8, wherein the thickness of said spacer layer is 20 μm or more.11. The information recording medium according to claim 8, wherein thereflectance of at least one of said first recording layer and saidsecond recording layer is 15% or more.
 12. The information recordingmedium according to claim 8, wherein the reflectance of at least one ofsaid first recording layer and said second recording layer is 45% orless.
 13. An information recording medium comprising: a supportingsubstrate; a first recordable or rewritable layer formed on saidsupporting substrate; a spacer layer formed on said first recordable orrewritable layer; a second recordable or rewritable layer formed on saidspacer layer; a cover layer formed on said second recordable orrewritable layer, wherein the thickness of the cover layer is smallerthan the thickness of the supporting substrate; and a protectiveover-coating layer formed on said cover layer and having a lightincident surface, wherein a light beam can enter through said lightincident surface and can be reflected to produce a reflected light beam,a recorded information is reproduced based on changes in light intensityof the reflected light beam and the reflectance of at least one of saidfirst recordable or rewritable layer and said second recordable orrewritable layer is 45% or less, wherein a recess or projection isformed inward of a recording area, and wherein the cover layer is formedradially outward of the recess or the projection and is not formedradially inward of the recess or the projection.